1. Field of the Invention
This invention relates generally to a two-component mixing type coating apparatus suitable for coating car bodies, machine components, and the like, by use of a paint consisting of a mixture of two components, that is, a main component and a hardener, such as a urethane paint.
2. Description of the Related Art
Coating of car bodies, machine components and the like, has been performed in the past by the use of a two-component type coating material. For this coating work, a coating supply apparatus has generally been employed which supplies a main component and a hardener from their supply sources to a coating spray gun 1 by operating respective pumps 2a and 2b as shown in FIG. 4 of the accompanying drawings, runs them together at a junction A in an intermediate portion of the supply conduit, mixes them together in a mixer 3 and supplies the mixture to the spray gun 1. In most cases, a check valve or valves 4a, 4b are disposed in one, or both, of the flow portions upstream of the junction A. During the supply of the main component and the hardener, particularly immediately after stopping the spray operation of the spray gun 1, the main component is likely to enter the flow portion of the hardener from the junction A, or vice versa, due to the difference of supply pressures between the main component and the hardener. The check valves 4a and 4b are provided in order to prevent such a backflow.
Generally, check valves have the following construction. As shown in FIG. 5, for example, a ball valve 6 and a coil spring 7 are held in a valve chamber 8 and a valve seat 9 for the ball valve is formed in such a manner that its diameter decreases gradually on the supply side of the valve chamber 8. There is also disposed a mechanism which pushes the ball valve 6 to the valve seat 9, such as coil spring 7. When no chemical is supplied, the valve is thus kept closed. When the main component or the hardener is supplied, the supply pressure of these chemicals moves the ball valve 6 away from seat 9 against the force of the coil spring 7. When the backflow P comes from the junction A, the pressure of the backflow pushes the ball valve 6 against the valve seat 9 in cooperation with the force of the spring 7, as represented by the dot-chain line in FIG. 5, so that the valve is closed and the backflow P is prevented from further entering the flow portion Q on the side of the supply source.
In accordance with the prior art apparatuses, however, when the coating spray gun 1 is repeatedly operated and stopped and the backflowing main component or hardener repeatedly enters the valve chamber 8 of the check valve, the main component and the hardener are mixed together and cured, and this cured product adheres to the coil spring 7 and the inner walls of the valve chamber 8 and limits the extension and contraction of the coil spring 7; that is, the valve opening and closing operation of the ball valve is deteriorated. As a result, the check valve is permanently retained in the state where it is somewhat open, as shown in solid lines in FIG. 5, and cannot be closed fully. Therefore, the backflow P unavoidably enters the flow portion Q on the supply source side.